Wavelet-based image coding for the purposes of compression and progressive transmission is the subject of many scientific papers, surveys and standardization initiatives. Well known methods include methods such as Embedded Zero-tree Wavelet (EZW) coding, Set Partitioning in Hierarchical Trees (SPIHT), and Embedded Block Coding with Optimized Truncation (EBCOT).
The EZW method exploits the self-similarity of a transformed image to enable an initial transmission of coarser scale coefficients, followed by the ordered transmission of refinement bits at successive bit plane levels. Coefficients deduced as zero in a particular pass need not be coded which ensures efficient compression when a zero tree is encountered. The EZW method typically requires an arithmetic encoder to process the symbols generated by the successive quantization process.
Various other wavelet coding methods use the concepts of EZW. SPIHT is a popular alternative that uses a different parent-child relationship to achieve a high performance even without an arithmetic encoder. However, SPIHT requires iterative pre-computation for the coding of each bit plane level which adds latency in a real time coding application. EBCOT, and related JPEG 2000-based coding, partitions each sub-band into blocks of samples and generates a separate scalable bit-stream to represent each code-block, thereby enabling selective decoding in applications seeking to improve resolution in only a portion of an image. Progressive Wavelet Coding (PWC) is an embedded coding method that uses adaptive run length encoding of bit planes of quantized wavelet transform coefficients to generate an embedded bit stream of bit-plane encoded macro-blocks that is scalable in resolution and fidelity. PWC uses explicit blocking of ordered coefficients to achieve resolution-based scalability and layering.
In summary, there are various wavelet coding techniques aimed at addressing the requirements for progressive image transmission. However, existing methods typically require incremental encoding of each bitplane and are unsuitable for applications in which an image changes rapidly and fast coding is desired. Therefore, there is a need for an improved method that is better suited to coding images for real-time progressive transmission.